This invention relates to the atomisation of certain combustible materials and also to their combustion thereafter.
The combustible materials envisaged in this invention are materials which are, or at a temperature up to 300.degree. C. are, a gas, a liquid, a particulate solid or a liquid containing particulate solids, and are, or at said temperature are, pumpable or flowable.
Such materials, for example gaseous or liquid fuels, only burn completely if they are mixed with a gas containing oxygen. Liquid fuel burners include essentially an atomiser. This atomiser is generally accommodated in the burner head, which is mounted at the extremity of the body or tube of the burner. The burner forms an integral part of a more or less complicated apparatus which includes suitable means for mixing the atomised fuel with a gas containing oxygen and for producing the desired flame.
Known burners make use of three atomising processes, atomising by an auxiliary fluid, mechanical atomising by the pressure of the fuel and atomising by rotating cup. Some burners embody a combination of these processes.
Atomising by an auxiliary gas uses the energy liberated by the expansion of compressed air or vapour under pressure for dispersing the fuel. This dispersion is produced by the meeting of a jet of liquid fuel and a jet of the auxiliary gas. The atomising obtained by this process is very coarse, above all in the case of the more viscous fuels. Moreover, the consumption of auxiliary fluid is quite considerable, generally in excess of 10 kg vapour to 100 kg heavy fuel oil. Consumptions of 15 to 20 kg vapour per 100 kg heavy fuel oil are normal.
In a burner with mechanical atomising by pressure, the fuel is injected tangentially into a cavity within which it performs a rapid rotary movement, while moving towards the orifice of the burner. This orifice is arranged in the center of a cap which covers the cavity and constitutes the burner jet. At the outlet of the jet or nozzle, the jet of liquid is in the form of a conical sheet. This type of atomiser only allows very limited variation in delivery. Various improvements, in particular the heavy oil return burner and the double feed burner, allow somewhat greater flexibility in operation; but these burners and their supply circuit are complex. All the mechanical atomising burners produce droplets which are larger the more viscous the fuel. The size of the drops also increases with the delivery. The nozzle, which is exposed to the radiation of the flame and the furnace, is frequently obstructed by the formation of coke.
Rotating cup burners are far less used than the previous ones. The cup, which is driven by compressed air or an electric motor, atomises the fuel by centrifuging.
If the problems inherent in the use of liquid fuels, such as heavy fuel oils, have found satisfactory solutions from a technical point of view whereas from an economic point of view these solutions are not free from imperfections, the same cannot be said of the problems raised by the combustion of by-products that very much more viscous than the usual fuel oils contain suspended solid constituents. For instance, it is not possible with any known burner to atomise certain tars and make them burn properly. Either the burner is obstructed too quickly, or the atomising of the fuel is far too coarse.
The present invention has as one object the overcoming of this problem whereby it is possible to burn fuels of very different kinds, only requiring a moderate consumption of auxiliary fluid and having great flexibility of operation as regards both the delivery of the fuel and its physical state.
In prior proposals for atomising by an auxiliary gas, the energy released by the expansion of the latter is first of all used to form a jet of gas in which this energy is converted into kinetic energy. Next, in the impact produced between the jet of gas and the jet of liquid fuel, a fraction of the kinetic energy of the two jets is used to overcome the forces of cohesion of the liquid. The efficiency of this process is poor for the mere impact of the jets only makes it possible to use a small fraction of the kinetic energy of the fluids to bring about the dispersion of the liquid. In other prior proposals, the initial jet of the auxiliary gas is produced by only a partial expansion of this gas and the mixture formed by the meeting of the two jets undergoes a further partial expansion or expansions, by passing through a narrow passage or several successive narrow passages, in which fresh divisions of the liquid drops are produced through shearing stresses. However, the greater part of the energy liberated under these conditions by the expansion of the gas is dissipated in heat, so that with burners that are thus improved, the consumption of auxiliary gas remains very considerable. Moreover, highly viscous fuels rapidly choke this type of burner.